From Seo Wiki - Search Engine Optimization and Programming Languages

The site, founded in 2002 by Bob Wyman and Salim Ismail, operated by storing a user's search term, making it a subscription, and checking it against posts on blogs which ping the search engine. When a new match was found, the user was notified, even if it occurred months after the initial search. This feature led PubSub to call itself a matching engine. It also checks incoming Usenet posts, USGS earthquake alerts, press release via NITF, SEC filings and FAA flight delay information.

Financial trouble

PubSub got into something of a problem as noted in this blog entry by Bob Wyman and now appears to be dead.

"Our days are numbered. A recent attempt to execute a merger has been blocked and we've been blocked from raising equity financing that would allow us to continue to pay salaries and pay off our $3 million in debt. Thus, our "doors" will close soon if we can't find someone to pull us out of the current situation. Persons with fast access to cash and a desire for some of the industry's best technology are advised to contact us rapidly...

The PubSub website was offline from 15 January 2007 until 15 August 2007. Its assets were purchased by Something Simpler. Something Simpler is preparing to relaunch the site as a user friendly version of Yahoo! Pipes. Something Simpler is planning to launch a Facebook application similar to the original PubSub sidebars.

As November 18, 2009. The PubSub v2.0 is officially public accessible again.